DESCRIPTION: This proposal seeks continued support for studies to characterize and apply DNA-mediated electron transfer. The study of the DNA p-stack as a medium for electron transfer is significant in understanding long range radical damage and repair, in characterizing long range charge transport through a p-stack, and in providing the basis for a new class of diagnostic tools. Results in the first grant period using metallointercalators as photoinduced donors and acceptors have established the critical requirement of intercalative p -stacking for fast electron transfer. Efficient photoinduced electron transfer has been demonstrated between metallointercalators tethered to either end of a 15-mer duplex (41u separation through the p-stack), and both solution and solid-phase methods have been developed to prepare a range of metal-oligonucleotide conjugates. Transient absorption spectroscopy has also been used in the DNA-mediated reaction to identify the electron transfer intermediate and, on an ultrafast timescale, to establish rates (1010 s-1). We now intend to apply the well characterized DNA polymer (i) to establish the factors influencing electron transfer processes through p-systems; (ii) to detail and establish the scope for the electron transfer chemistry by varying intercalating donors and acceptors; and (iii) to exploit the p-stack in developing new routes to DNA-based sensors. We will delineate the parameters for the long range reaction in measurements of electron transfer rates on a series of duplex assemblies containing tethered metallontercalators, which vary in DNA sequence, length, and conformation. Rates will also be compared in a duplex constructed through hybridization of two singly metallated oligonucleotides and a duplex doubly metallated on one strand. In characterizing the electron transfer chemistry, metallointercalators will be varied with respect to core metal, ancillary ligands, and stereochemistry to determine the effects of driving force and stacking on rate. Organic intercalators will be employed to establish the scope of this fast reaction and to provide spectroscopic markers. A chemical trap is also proposed, involving the repair of a thymine dimer site- specifically incorporated in an oligonucleotide by photooxidation with a rhodium intercalator. Electron transfer and exchange energy transfer will be compared directly in mixed metal-DNA assemblies constructed with Ru(II)/Os(III) versus Ru(II)/Os(II). The development of new luminescent sensors, sensitive to perturbations in base pair stacking, are proposed using the DNA helix as a "wire". Assemblies will be constructed to detect base pair mismatches and to monitor changes in stacking associated with protein binding (HaeIII, EcoRI, and the TATA-box binding protein) and with coordination of the antitumor agent, cis- diamminePt(II). This electron transfer detection strategy provides a completely new basis for the development of DNA diagnostics and for probing the DNA p-stack in solution.